Pulley assemblies comprising a hub adapted to be rigidly connected to a crankshaft of an internal combustion engine and a pulley adapted to cooperate with a belt of the accessory drive and rotationally connect to the hub by means of a spiral spring are known.
Such known assemblies also generally comprise an integrated torsional damper, which consists of an annular seismic mass coaxial to the hub and an elastic connection element arranged between the hub and the seismic mass.
In use, the spiral spring is coupled with the rotation of the hub and the pulley and presents a relatively low stiffness so as to allow to filter the torsional vibrations of the crankshaft and avoid such vibrations from being transmitted to the accessory drive.
In the known pulley assemblies, the pulley is connected radially to the hub by means of bushing and comprises a cylindrical wall defining a central cavity for accommodating a portion of the hub and the spiral spring. The spiral spring comprises a first end portion connected rigidly to the hub and a second end portion fitted with radial interference against the cylindrical wall.
During operation, the pulley can advance the hub and the second end portion may slide in relation to the cylindrical wall when a maximum transmissible torque value is exceeded. Such condition occurs during cranking of the engine when there are considerable fluctuations in torque, or during the instants immediately following sudden deceleration when the crankshaft turns at a relatively slow angular speed while the accessory drive proceeds by inertia at a higher angular speed.
In the known pulley assemblies, the resultant of the radial pressures acting between the second end portion of the spring and the cylindrical wall causes an additional stress on the bushing which tends to wear out and have a limited working life.